There has been a rapid growth of terrestrial and satellite communication systems in recent years. This growth has been accompanied by an increasing need for systems with higher information-carrying capacities within limited frequency bands. Since free space enables the independent and simultaneous transmission of electromagnetic waves having polarizations which are orthogonal to one another within the same frequency band, the use of orthogonal polarizations can double the information-carrying capacity of a communications link. However, rainfall, imperfect antenna alignment, multipath fading, terrain reflections and atmospheric phenomenon diminish the isolation between the orthogonally polarized signals. This results in cross-coupling interference. Consequently, at the system receiver, information carried by one designated polarization is corrupted with information carried by the orthogonal polarization. This cross-coupling interference can degrade the quality of the received information below that which is acceptable in many telecommunications applications.
Several techniques for cross-polarization interference cancellation have been proposed. See, for example, an article by M. L. Steinberger entitled "Design of a Terrestrial Cross-Pol. Canceller", International Conference on Communications, June 13-17, 1982, Philadelphia, Pa., and the canceller suggested in the articles of Y. Bar-Ness et al, "Cross-Coupled Boot Strapped Interference Canceller", IEEE 1981 International Symposium Digest, Antennas and Propagation, Vol. 1, pp. 292-5, and D. Brandwood, "Cross-Coupled Cancellation System for Improving Cross-Polarisation Discrimination", International Conference on Antennas and Propagation, Part 1, pp. 41-5, 1978. While the proposed schemes referred to in these articles should provide satisfactory cross-polarization interference cancellation in certain situations, they may provide poor performance during frequency-selective fading and are not usable in nonregenerative repeaters. In addition, the effectiveness and speed of the cancellation process are insufficient for certain system applications and modulation formats.